An emulsion is one of the possible outcomes of mixing two liquids. Here is the emulsion definition, examples, types of emulsions, and a look at their uses.
An emulsion is defined as a mixture of two or more normally immiscible (unmixable) liquids. Emulsions are colloids, which are homogeneous mixtures consisting of particles larger than molecules that scatter light, but are small enough that they don’t separate. Emulsions consist of two parts: the dispersed phase and the dispersing medium (continuous phase).
The word emulsion comes from the Latin word emulgere, which means “to milk.” Milk is an emulsion of fat and other components dispersed in water. Emulsification is the process of turning a liquid mixture into an emulsion.
Types of Emulsions
Emulsions are classified according to the nature of the dispersed phase and dispersing medium (also called the dispersion medium or continuous phase):
- Oil in water (O/W): An O/W emulsion consists of oil (or other nonpolar liquid) dispersed in water. Milk is a good example of an O/W emulsion, as it consists of fat globules (dispersed phase) in water (dispersing medium).
- Water in oil (W/O): A W/O emulsion consists of water dispersed in oil. Butter and margarine are examples of W/O emulsions. A small amount of water is dispersed in a larger volume of oil.
Emulsions are common in cooking and everyday products. Examples include:
- Oil and water, when vigorously shaken
- Egg yolk (water and fat emulsified by lecithin)
- Vinaigrette (an emulsion of oil and water)
- Butter (an emulsion of water in fat)
- Mayonnaise (oil in water stabilized by lecithin in egg yolk)
- Many moisturizers (either oil in water or water in oil)
- Crema on espresso (coffee oil in water)
- Hollandaise sauce
Although the photosensitive side of photographic film is called an emulsion, it technically isn’t one because the phases aren’t liquids. Photographic emulsion is silver halide in gelatin.
Properties of Emulsions
Most emulsions look cloudy or white because the phase interphases between the components of the mixture scatter light. Dilute emulsions may appear slightly blue due to the Tyndall effect. Skim milk is an example of a dilute emulsion. Microemulsions and nanoemulsions consist of particles smaller than 100 nm in diameter, which are too small to scatter light. These emulsions may appear clear.
Because they consist of liquids, emulsions lack ordered internal structure. The droplets are more or less evenly distributed throughout the dispersion medium, but they don’t appear uniform in size under magnification.
The components of emulsions don’t spontaneously mix. Energy is required, usually in the form of shaking, stirring, or ultrasonic homogenization. If an emulsifier is used, two immiscible liquids may form a stable emulsion that doesn’t separate out over time. Otherwise, emulsions eventually revert to their original phases.
Emulsifiers and Emulsifying Agents
An emulsifier, emulsifying agent, or emulgent is a substance that stabilizes an emulsion. An emulsifier can be cationic, anionic, or nonpolar, but it has both a hydrophilic (nonpolar) and hydrophobic (polar) portion. This makes it soluble in both oil and water.
Whether an emulsion is O/W or W/O isn’t just a matter of which component is present in the greatest proportion. The emulsifier also affects the type of emulsion that forms. Emulsifiers that are more soluble in water than in oil allow water to act as the dispersion medium, forming an oil in water dispersion. Proteins, soaps, and detergents are common emulsions for making O/W emulsions. Emulsifiers that are more soluble in oil form water in oil emulsions. Examples of these substances include long-chain alcohols and fatty acid metal salts.
Examples of common emulsifiers are:
- Soy lecithin
- Egg yolk (which contains lecithin)
- Sodium phosphate
- DATEM (diacetyl tartaric acid ester of monoglyceride)
- Sodium stearoyl lactylate
Difference Between Emulsion and Colloid
Sometimes the words “emulsion” and “colloid” are used interchangeably, but they don’t mean quite the same thing. An emulsion is a type of colloid. A colloid, in turn, is a type of homogeneous mixture. All emulsions are colloids, but not all colloids are emulsions. An emulsion is a colloid in which all the phases are liquids. There are other types of colloids, defined according to their phases. For example, an aerosol is a solid dispersed in a gas (e.g., smoke), while a foam is as gas dispersed in a liquid (e.g., whipped cream).
How Emulsification Works
Emulsification occurs via several mechanisms.
- Mechanical mixing breaks liquids into smaller particles, so it takes longer for the components to separate. For example, if you shake a bottle of oil and water, it separates pretty quickly. If you use a blender, the emulsion takes longer to revert to its previous state.
- Some emulsifiers reduce the interface surface tension between the two liquids, allowing them to mix. Surfactants work using this principle.
- Some emulsifiers encapsulate or form a film over one component of a mixture. The coated particles repel each other, so the contents remain evenly dispersed.
- An emulgent may increase liquid viscosity so it’s easier for particles to remain suspended. Thickeners form emulsions this way. Examples includes acacia, glycerin, tragacanth, and carboxymethyl cellulose.
How to Separate Emulsion Components
Some emulsions separate on their own, but others are fairly stable. Emulsions made using emulsifiers may be separated using temperature and gravity. Techniques include:
Emulsions have many uses:
- In cooking, emulsions find use in sauces, ice cream, and baked goods. Some raw ingredients are emulsions, including milk, butter, and eggs.
- Emulsions are using in cosmetics, personal hygiene products, and drugs.
- Glue, paste, and paint use emulsions to mix dissimilar ingredients.
- Microemulsions deliver some vaccines.
- Microemulsions kill some pathogens by disrupting their cell membranes.
- Some fire extinguishers (for class B fires) use emulsifying agents to trap flammable vapors with water.
- Emulsions are used to make synthetic latex and other polymers.
- IUPAC (1997). “Emulsion”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Blackwell Scientific Publications: Oxford. ISBN: 0-9678550-9-8. doi:10.1351/goldbook
- Khan, A. Y.; Talegaonkar, S; Iqbal, Z; Ahmed, F. J.; Khar, R. K. (2006). “Multiple emulsions: An overview”. Current Drug Delivery. 3 (4): 429–43. doi:10.2174/156720106778559056
- Levine, Ira N. (2001). Physical Chemistry (5th ed.). Boston: McGraw-Hill. p. 955. ISBN 978-0-07-231808-1.
- Silvestre, M.P.C.; Decker, E.A.; McClements, D.J. (1999). “Influence of copper on the stability of whey protein stabilized emulsions”. Food Hydrocolloids. 13 (5): 419. doi:10.1016/S0268-005X(99)00027-2